1. Field of the Invention
This invention relates to electrical interfacing board apparatus for effecting concurrent, respective, forceful engagements to make electrical connections with each of a multiplicity of generally coplanar, electrically conductive zones arranged in a permissably irregular pattern upon an electrical device such as, for example, a printed circuit board to be tested.
The invention is particularly concerned with providing an improved construction for such interfacing apparatus of type adaptable for use in effecting concurrent, electrical connections with the multitudinous, densely spaced and typically irregularly arranged test point zones of modern printed circuit boards, of configuration permitting the employment of the same interfacing board in the testing of printed circuit boards having differing patterns of test point zones, and, perhaps most significantly, of nature providing the required strength, precision and reliability while remaining technically and economically practicable to produce.
2. Description of the Prior Art
It has long been the common practice in the testing of printed circuit boards and similar devices to use some form of interfacing apparatus for effecting the normally temporary, electrical connections required to be made between the electrical circuitry of the testing equipment and the various electrically conductive zones of the device constituting electrical test points. Such interfacing apparatus has typically employed a plurality of contactor assemblies, usually of a spring biased pin type, for establishing electrically contacting engagement with each of the test point zones of the device under test (or some intermediate transition assembly for accomplishing a physical coordinate conversion), with each of such contactors being coupled by a wire or the like to the testing equipment circuitry.
An early practice in such interfacing apparatus was simply to provide one contactor assembly for each test point to be engaged and to mount such contactors in appropriate locations on a board or carrier assembly for aligning with and engaging the test point zones on a particular type of printed circuit boards when the contactor carrying board was suitably juxtaposed with the device being tested. Since the test point zones on a printed circuit board are typically arranged in an irregular pattern, this early approach necessitated the drilling of mounting holes for the contactor assemblies in the carrier board in an irregular pattern also. Another disadvantage of such approach was that, since the pattern in which the contactor assemblies were arranged and mounted on the carrier board was usually irregular and matched to the test point pattern of one particular type printed circuit board for which the interfacing apparatus was specifically designed, it was necessary to provide different interfacing apparatuses for each type of printed circuit board to be tested and to substitute a different interface apparatus in the testing fixture each time a different type of device was to be tested.
Because of the time, cost and inconvenience factors inherent in the above-mentioned early approach, effort was then directed to providing interfacing apparatus including an intermediate transition assembly that would accomplish physical coordinate conversion between the irregularly arranged test point zones of a particular type of printed circuit board and a set of logically corresponding contactor assemblies mounted in a regular pattern such as at the intersection points of a matrix. The Wickersham U.S. Pat. No. 3,654,585 illustrates this technique and disclosed an implementation thereof in which the transition assembly was in the nature of a board to be interposed between the circuit board under test and an array of regularly arranged contactor assemblies; the intermediate board was itself fabricated in a manner analogous to that employed in printed circuit boards, was provided on one face thereof with a set of contacts arranged in an irregular pattern for engaging the test point zones of the type of device to be tested, was provided on the opposite face thereof with a regularly arranged pattern of contacts to be engaged by the array of contactor assemblies, and was further provided with electrically conductive means passing through the transition board for interconnecting logically corresponding contacts on the opposite faces thereof. Subsequently, a refined implementation of such technique was commercialized in which the opposed contacts and electrical interconnections therebetween associated with the transition board were provided by bent electrically conductive pins reciprocably carried by the transition board with one end of the pins arranged in an irregular pattern to match the pattern of test points on the type of device to be tested and the other end of such pins regularly arranged in a matrix for engagement by the array of spring pin contactor assemblies. Such coordinate conversion transition assembly technique permitted the more economical fabrication of interfacing apparatus, since the spring pin contactor assemblies with which individual wires leading to the test equipment must be associated would be arranged and mounted in a regular matrix pattern. However, even with that improvement, the problem of needing to provide a different form of transition assembly for each type of printed circuit board or similar device to be tested still remained, and, although such technique has proved quite satisfactory in applications where relatively large quantities of only a limited number of specific device types are to be tested during a given time period, the expenses involved in providing differing transition assemblies for each type of device to be tested, as well as the need for substituting different transition assemblies in the testing fixture, continued to present a substantial economic burden for applications involving the testing of relatively small quantities of each of a number of types of custom designed printed circuit boards.
Accordingly, it has been recognized and found in practice that the over-all economies and convenience considerations in connection with the provision of interfacing apparatus for use in testing differing types of printed circuit boards or the like could best be served by providing the interfacing apparatus with an array of contactor assemblies arranged in a regular matrix pattern for engaging the printed circuit board under test at a multiplicity of regular intervals across the surface of the latter, even though only particular ones of such contactor assemblies would be engaging test point zones, while the others would be redundant insofar as the testing of each specific type of printed circuit board devices is concerned. With such last mentioned approach, however, one form of interfacing assembly can be employed in testing a diverse variety of specific types of printed circuit boards. Moreover, with the sophisticated circuitry available in the testing equipment typically employed in such applications, it is a simple matter for the testing equipment to electrically select which contactor assemblies it needs to be electrically coupled with for purposes of testing each specific type of printed circuit board to be handled. It will also be apparent that considerable time, expense and inconvenience are saved by this general approach by virtue of permitting the drillings necessary for the mounting of the contactor assemblies to be carried out in a regular pattern, rather than in an irregular pattern as required when each contactor assembly must be located to match the position of a corresponding test point zone on a device to be tested.
Concurrent development of the technology of fabrication of printed circuit boards themselves has, however, added additional and subtle aspects to the problem of providing a full solution to the problem. Briefly, the degree of both complexity and miniaturization of circuit paths on printed circuit boards have increased substantially, while the areas of circuitry on such devices have tended to remain the same or even increase, with the result that a typical modern printed circuit board presents a much larger multiplicity of test points with which electrical connections must be made during testing than was the case with earlier such devices, and such test point zones also tend to be smaller and at closer intervals than was the case with earlier such devices. This, in turn, necessitates the employment, with any interfacing apparatus which is to be adapted for use with diverse types of printed circuit boards, of a much larger multiplicity of contactor assemblies at much closer spacings than would have been required for the testing of most earlier types of printed circuit boards.
The increased number and density of contactor assemblies required in the mentioned type of regular array or matrix of same for use in testing modern printed circuit boards unexpectedly gives rise, however, to problems having mechanical as well as electrical implications in connection with the practical implementation of a type of apparatus that has heretofore been primarily thought of as being of electrical character. The more closely spaced drillings required for the mounting of dense matrix of spring pin contactor assemblies in an electrically insulative carrier board has significant influence upon the physical strength of that board. The engagement force required between the contacting portion of a spring pin contactor assembly and the corresponding test point zone on a device under test, in order to effect a sufficiently reliable and low electrical resistance, electrical contact therebetween, is preferably in the range of about 2-8 ounces per contactor assembly. Considering that a matrix interval of 0.05 inch between adjacent contactor assemblies will dispose about 400 of the latter within each square inch of the carrier board, it will be perceived that a force of between about 50 lbs. per square inch and 200 lbs. per square inch will be exerted back upon the carrier board in the direction of its thickness by the contactor assemblies when they are in operative engagement with a device under test. When it is further recognized that the contactor assembly carrier board of interfacing apparatus will normally be of length and width dimensions of at least several inches each in order to be employed in testing typical types of printed circuit boards, and that such contactor assembly carrier boards must normally be supported primarily along marginal portions thereof in order to provide clearance for egress of the multitude of wires leading from the individual contactor assemblies to the testing equipment, it will be appreciated that the contactor assembly carrier board or other assemblage utilized for that purpose must be able to withstand very substantial, aggregate, physical forces exerted in what would normally be its direction of greatest weakness, not only to avoid possible physical breakage, but also to minimize distortion that could adversely affect either the alignment thereof required for engagement with small test point zones of the device under test or altered electrical relationships between the already closely spaced contacting portions of the contactor assemblies which might adversely influence the testing results.
It might seem that a solution to the problem would be provided merely by increasing the thickness of the electrically insulative plate to an extent sufficient to provide the requisite physical strength. However, that approach has been found to be impractical in connection with the drilling of the holes through the contactor assembly carrier board required for mounting of the spring pin contactor assemblies and providing clearance for the connection wires individually leading therefrom, both by virtue of the tendency of drill bits to wander or deviate from their intended straight course when drilling in very thick plates of material (and especially the electrically insulative fiber glass material commonly employed in fabricating such boards), as well as the increased tendency toward breakage of the very fine bits necessarily employed, when attempting to drill through a very thick plate.
Again, it might appear that a satisfactory solution would be available through the mere expedient of separately drilling and then stacking a number of thinner plates having corresponding holes therein aligned to present the required physical strength in the composite carrier board assembly. It has been found, however, that even that approach does not alone provide a fully satisfactory solution. First, bearing in mind that a typical interfacing assembly for use in testing printed circuit boards of, say, 12 inch by 12 inch dimensions, would require the drilling of over 57,000 holes in each plate in order to provide for a matrix of contactor assemblies mounted at 0.05 inch intervals, it will be apparent that the amount of precision drilling required can compound very quickly with the number of plates to be utilized in a composite carrier board assembly. Secondly, with the high density of holes in such an assembly, it has been found that the physical strength of the individual plates, and thereby the composite assembly, tend to be weakened to an unexpectedly significant degree, thus leading to the necessity for employing an undesirable number of plates and the concomitant multiplication of the expensive, precision drilling operations required.
It is in this general context of previously inadequately practical solutions to the problem of providing economical, reliable, precision and versatile interfacing apparatus that the improved construction contemplated by this invention has been conceived and developed.